In the bowl of an electric mixer, combine the softened butter, zest, sugar, salt, and vanilla extract and beat with the paddle until light and fluffy. Add the egg and beat it in well. In three additions, stir in the flour until blended. Spread the dough out, about an inch thick, on a sheet pan, cover with plastic wrap, and chill until firm. You can refrigerate the dough, wrapped well, for up to a week.

Roll the chilled sugar cookie dough between two pieces of parchment into a rectangle until it's between 1/8 and 1/4 inch thick. Remove the top piece of parchment and flip the dough into the prepared half sheet pan so that it fits along the bottom and up the sides completely. If the dough cracks a bit, just press it back together. Press it into the sides of the pan; don't leave any gaps. Cut off any excess dough by running a rolling pin along the edges of the pan. Chill until firm.

Heat the oven to 350 degrees F. Line the dough with foil or parchment and weight it with dried beans or pie weights. Bake until the edges are golden, about 20 minutes. Remove the weights and the foil or parchment and continue to bake until completely set, about another 10 minutes. Let cool before filling.

In a large heavy-based saucepan, combine the sugar, corn syrup, and water and bring to a boil. Cook until the mixture becomes amber in color (approximately 350 degrees F to 360 degrees F on a candy thermometer). Immediately remove from the heat. Carefully add the butter and honey, return to the heat, and stir until dissolved. Bring the mixture back to a boil and carefully add the heavy cream and zest (the mixture will bubble up and may splatter). Boil the mixture until it reaches 250 degrees F on the candy thermometer. Remove from the heat and stir in the almonds (and candied citrus peel, if using). Quickly pour the mixture into the baked sugar cookie shell before the caramel cools. Spread the nut mixture evenly with a lightly greased spatula.

Bake until the topping begins to bubble, 18 to 20 minutes. Put the pan on a wire rack and let cool completely. Cut along the edge of the pan to loosen the edges. Turn the Florentines out, upside down, onto a clean cutting surface. Line two clean baking sheets with parchment.

With a serrated bread knife, score the ragged edges of the cookie crust, cutting through the crust but not the filling. With a large chef's knife, cut through the filling to trim off the ragged edges completely. Next, using the serrated knife, score 1-1/4-inch-wide horizontal bands top to bottom (again deeply, through the crust but not through the filling). To make the diamonds, score 1-1/4-inch strips starting at the top left corner and dividing that corner into two 45 degree angles. Continue scoring at this width and this angle until all the cookies are scored.

Follow the scoring lines with the chef's knife, using steady force to cut through the thick nut filling.

Turn each Florentine over and set on the clean sheet pans. Fill a pastry bag with a tiny tip (or a heavy plastic bag with a corner cut off, or a paper cone) with the melted chocolate (or use a fork dipped in the chocolate) and drizzle it on the diamonds. The cookies are best at room temperature but can be refrigerated if the kitchen is too warm for the chocolate to set.